BTreeMap: fix minor testing mistakes in #78903

Mostly a duplicate test case
r? `@Mark-Simulacrum`

Add test for eval order for a+=b

Yes, the order of evaluation *does* change depending on the types of
the operands. Cursed, I know.

I've elected to place this test into `expr/compound-assignment` creating
both the `expr` directory and the `compound-assignment` directory. I
plan in a future PR to also move the `if` directory and the loose `if`
tests into `expr/if` and other similar cleanups of the `test/ui`
directory.

Future work: Test more than just `+=`, but all operators. I don't know
if using a macro to generate these tests cases would be okay or not,
but it'd be boilerplatey without it. I'm also confident you cannot
change the evaluation order of one operator without changing all of
them.

Future work: Additionally, test more than just `i32 += i32` for the
primitive version. I don't actually know the full set of primitive
implementations, but I imagine there's enough to cause a combinatorial
explosion with the previous future work item. Somewhere on the order of
one to two hundred individual functions.

BTreeMap: address namespace conflicts

Fix an annoyance popping up whenever synchronizing the test cases with a version capable of miri-track-raw-pointers.

r? `@Mark-Simulacrum`

Clean up `StructuralEq` docs

Stabilize refcell_take

Tracking Issue: #71395

``@KodrAus`` nominated this for FCP, so here's a PR!
I've never made a stabilization PR, so please mention if there's anything I can improve, thanks.

Stabilize alloc::Layout const functions

Stabilizes #67521. In particular the following stable methods are stabilized as `const fn`:

* `size`
* `align`
* `from_size_align`

Stabilizing `size` and `align` should not be controversial as they are simple (usize and NonZeroUsize) fields and I don't think there's any reason to make them not const compatible in the future. That being true, the other methods are trivially `const`. The only other issue being returning a `Result` from a `const fn` but this has been made more usable by recent stabilizations.

Split each iterator adapter and source into individual modules

This PR creates individual modules for each iterator adapter and iterator source.

This is done to enhance the readability of corresponding modules (`adapters/mod.rs` and `sources.rs`) which were hard to navigate and read because of lots of repeated lines (e.g.: `adapters/mod.rs` was 3k lines long). This is also in line with some adapters which already had their own modules (`Flatten`, `FlatMap`, `Chain`, `Zip`, `Fuse`).

This PR also makes `Take`s adapter fields private (I have no idea why they were `pub(super)` before).

r? ``@LukasKalbertodt``

Add f{32,64}::is_subnormal

The docs recommend that you use dedicated methods instead of calling `classify` directly, although there isn't actually a way of checking if a number is subnormal without calling classify. There are dedicated methods for all other forms, excluding `is_zero` (which is just `== 0.0` anyway).

Stabilizes #67521. In particular the following stable methods are stabilized as const fn:

* size
* align
* from_size_align

Consolidate exhaustiveness-related tests

I hunted for tests that only exercised the match exhaustiveness algorithm and regrouped them. I also improved integer-range tests since I had found them lacking while hacking around.
The interest is mainly so that one can pass `--test-args patterns` and catch most relevant tests.

r? `@varkor`
`@rustbot` modify labels: +A-exhaustiveness-checking

Rollup of 4 pull requests

Successful merges:

 - #78670 (Remove FIXME comment in some incremental test suite)
 - #79292 (Fix typo in doc comment for report_too_many_hashes)
 - #79300 (Prevent feature information to be hidden if it's on the impl directly)
 - #79302 (Add regression test for issue 73899)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Add regression test for issue 73899

Closes #73899

Prevent feature information to be hidden if it's on the impl directly

Fixes #79279.

So when a `#[doc(cfg...)]` is used on a trait impl directly, it's not hidden by the toggle.

![Screenshot from 2020-11-22 14-40-11](https://user-images.githubusercontent.com/3050060/99905528-997fab00-2cd1-11eb-83be-ed06b0cb97df.png)

r? `@jyn514`

Fix typo in doc comment for report_too_many_hashes

"to big" -> "too big"

Remove FIXME comment in some incremental test suite

Helps with #62277

I removed FIXME comment in some incremental tests with [rustc_partition_codegened](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_incremental/assert_module_sources/index.html). This seems using codegen process. So it uses intentionally `build-pass`

Adds regression test for https://github.com/rust-lang/rust/issues/73899

<[T]>::reverse: Fix pointer provenance rules

Should fix #78749

Yes, the order of evaluation *does* change depending on the types of
the operands. Cursed, I know.

I've elected to place this test into `expr/compound-assignment` creating
both the `expr` directory and the `compound-assignment` directory. I
plan in a future PR to also move the `if` directory and the loose `if`
tests into `expr/if` and other similar cleanups of the `test/ui`
directory.

Future work: Test more than just `+=`, but all operators. I don't know
if using a macro to generate these tests cases would be okay or not,
but it'd be boilerplatey without it. I'm also confident you cannot
change the evaluation order of one operator without changing all of
them.

Future work: Additionally, test more than just `i32 += i32` for the
primitive version. I don't actually know the full set of primitive
implementations, but I imagine there's enough to cause a combinatorial
explosion with the previous future work item. Somewhere on the order of
one to two hundred individual functions.

Stabilize clamp

Tracking issue: https://github.com/rust-lang/rust/issues/44095

Clamp has been merged and unstable for about a year and a half now. How do we feel about stabilizing this?

"to big" -> "too big"

More consistently use spaces after commas in lists in docs

This PR changes instances of lists that didn't use spaces after commas, like `vec![1,2,3]`, to `vec![1, 2, 3]` to be more consistent with idiomatic Rust style (the way these were looks strange to me, especially because there are often lists that *do* use spaces after the commas later in the same code block 😬).

I noticed one of these in an example in the stdlib docs and went looking for more, but as far as I can see, I'm only changing those spots in user-facing documentation or rustc output, and the changes make no semantic difference.

Add "memset" as doc alias to slice::fill()

Similar to 53f969df and should make it easier for people coming from C to find this function.

Give a better error when rustdoc tests fail

- Run the default rustdoc against the current rustdoc
- Diff output recursively
- Colorize diff output

Closes https://github.com/rust-lang/rust/issues/78750.

## Resolved questions

- Should this be opt-in instead of on by default?
  + No
- Should this call through to `delta`? That's not a very common program to have installed, but I'm not sure how to do diffs after the fact. Maybe `compiletest` can take a `--syntax-highlighter` parameter or something?
  + I decided to use `delta` if available and `diff --color` otherwise. It prints a warning if delta isn't installed so you know you can get nicer diffs

## Open questions.

- What version of rustdoc would this compare against? Ideally it would compare against `$(git merge-base HEAD origin/master)` - maybe that's feasible if we install those artifacts from CI?
- Does it always make sense to compare the tests? Especially for new tests, I'm not sure how useful it would be ... but then again, one of the questions I want to know most as a reviewer is 'did it break before?'.

r? `@GuillaumeGomez`
cc `@Mark-Simulacrum`

Some UI tests started failing after moving iterator adapters to different modules.

This commit also makes fields of `Take` private. I have no idea why they
were `pub(super)` before.

Add support for custom allocators in `Vec`

This follows the [roadmap](https://github.com/rust-lang/wg-allocators/issues/7) of the allocator WG to add custom allocators to collections.

r? `@Amanieu`

This pull request requires a crater run.

### Prior work:
- #71873: Crater-test to solve rust-lang/wg-allocators#1
- [`alloc-wg`](https://github.com/TimDiekmann/alloc-wg)-crate

Rollup of 8 pull requests

Successful merges:

 - #77844 (clarify rules for ZST Boxes)
 - #79067 (Refactor the abi handling code a bit)
 - #79182 (Fix links to extern types in rustdoc (fixes #78777))
 - #79231 (Exhaustively match in variant count instrinsic)
 - #79238 (Direct RUSTC_LOG (tracing/log) output to stderr instead of stdout.)
 - #79256 (Fix typos)
 - #79264 (Get rid of some doctree items)
 - #79272 (Support building clone shims for arrays with generic size)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Support building clone shims for arrays with generic size

Fixes #79269.

Get rid of some doctree items

They can be derived directly from the `hir::Item`, there's no special logic.

- TypeDef
- OpaqueTy
- Constant
- Static
- TraitAlias
- Enum
- Union
- Struct

Part of #78082 (the easiest part, I'm still debugging some other changes).
r? `@GuillaumeGomez`

Fix typos

Direct RUSTC_LOG (tracing/log) output to stderr instead of stdout.

Looks like this got missed in the initial implementation, AFAIK the old behavior was to output on stderr.
(Hit this while trying to debug `rustc` running inside a build script which was only letting stderr through)

r? ``@oli-obk`` cc ``@davidbarsky`` ``@hawkw``